1. Field of the Invention
The present invention relates to micro aspherical lenses and a fabricating method therefor, and optical fiber coupling devices, focusing optical systems, optical devices, semiconductor laser light sources and image devices utilizing the micro aspherical lenses as well as multi-beam projectors.
2. Description of the Background Art
Micro lenses have the advantage of compactness and light weight. Micro lenses heretofore known include (a) a ball lens, (b) an ion exchange flat micro lens, and (c) a Fresnel lens. In addition, aspherical lenses have the advantage of causing no aberration. Aspherical lenses heretofore known include, for example, (d) an aspherical plastic lens used in an optical pick-up of an optical disk. However, the lenses have various disadvantages as described in the following.
(a) Ball Lens
The lens causes on-axis aberration. Accordingly, if the lens is used for optical coupling between an optical fiber and a semiconductor laser and between optical fibers, optical coupling efficiency is reduced. In addition, it is difficult to apply the lens to a precision optical system such as an optical pick-up.
(b) Ion-Exchange Flat Micro Lens
The lens is fabricated by ion exchange on a glass substrate. Accordingly, it takes approximately ten hours to fabricate the lens. In addition, the lens is not suitable for quantity production because it can not be reproduced.
(c) Fresnel Lens
The lens utilizes diffraction of light. Accordingly, the lens causes chromatic aberration in a light source where wavelength dispersion occurs.
(d) CD Aspherical Plastic Lens
The lens is 4 to 5 mm in diameter and 3 to 4 mm in thick. Accordingly, it is difficult to miniaturize, integrate and hybridize an optical system.
On the other hand, optical systems used in a multi-beam projector for projecting a lot of spotlights caused by diffraction, for example, a multi-beam projector used for shape recognition, for forming a multi-beam spot caused by diffraction include, for example, a fiber grating and an amplitude modulation grating, and an incoherent type lens array utilizing a focusing spot.
FIGS. 23, 24a and 24b illustrate a fiber grating. The fiber grating is constructed by arranging a plurality of optical fibers 81 in parallel with each other and arranging a plurality of optical fibers 82 in parallel with each other in the X direction so as to be overlapped with the optical fibers 81. In the fiber grating, however, the position of spots formed by light converged in the Y direction by the optical fibers 81 and the position of spots formed by light converged in the X direction by the optical fibers 82 are shifted by .SIGMA.f, as shown in FIGS. 24a and 24b. Therefore, the spots are blurred, thereby causing diffracted light spots P.sub.S2 to be blurred.
FIG. 25 illustrates an amplitude modulation grating. An opaque flat plate 83 is provided with holes 83a arranged at constant intervals, to obtain light spots P.sub.S3 utilizing diffraction of light which is transmitted through the holes. In the amplitude modulation grating, light which is not transmitted through the opaque flat plate 83 can not be utilized, so that use efficiency of light is low.
FIG. 26 illustrates an incoherent type lens array, which is constructed by arranging convex lenses 84 having a diameter of several millimeters or less in a two-dimensional manner. A lot of light spots P.sub.S4 are formed on the focal plane of the convex lenses 84, so that the focal depth of the spots is small.
Additionally, optical systems used for shape recognition include a cylindrical lens 85 as shown in Fig. 27. In the optical system using the cylindrical lens 85, an object to be detected 86 is illuminated by slit light Accordingly, only a two-dimensional shape can be recognized. In addition, if the object 86 to be detected is displaced in the Z direction (the direction of the principal axis), the position illuminated by the slit light is changed.